Despite having gone their separate ways at least a
billion years ago, plants and animals have developed remarkably
similar mechanisms for detecting the molecular signatures of
infectious organisms.Courtesy of Tree of Life Web Project

Though it's been at least a billion years since plants and animals shared
a common ancestor, they have through the eons shared a common threat in the
form of microbes, including bacteria, eukaryotes and viruses. This has
resulted in remarkably similar mechanisms for detecting the molecular
signatures of infectious organisms that hold promise for the future
treatment of infectious diseases in humans.

The recognition of microbial signature molecules by host receptors is the
subject of a paper published in the journal Science titled "Plant
and Animal Sensors of Conserved Microbial Signatures." The corresponding
author of the paper is Pamela Ronald, a plant pathologist who holds joint
appointments with the U.S Department of Energy (DOE)'s Joint BioEnergy
Institute, where she serves as Vice President for the Feedstocks Division
and directs the grass genetics program, and with the University of
California (UC) Davis, where she is a professor of plant pathology.
Co-authoring the paper with Ronald was Bruce Beutler, an immunologist and
mammalian geneticist with the Scripps Research Institute.

"If evolution is depicted as a tree, and extant species as terminal
leaves on that tree, we must acknowledge that we have examined only a few of
those leaves, gaining only a fragmentary impression of what is and what once
was," Ronald says. "In the future, a diverse array of evolutionarily
conserved signatures from pathogenic microbes will likely be discovered and
some of these will likely serve as new drug targets to control deadly groups
of bacteria for which there are currently no effective treatments."

In the Science paper, Ronald describes how the long-held
presumption that the mechanisms of plant and animal defense against microbes
are separate and distinct has undergone a complete change. "Discoveries over
the past 15 years demonstrate that the mechanisms that allow plants and
animals to resist infection show impressive structural and strategic
similarity," Ronald says. "We now know that plants and animals respond to
microbial signature molecules using analogous regulatory modules, which
likely came about as a consequence of convergent evolution."

While host sensor–mediated immune responses are essential for innate
immunity in both plants and animals, sustained or highly induced immune
responses can be harmful, which makes negative regulation of these pathways
critical. In animals, negative regulators act at multiple levels within
certain molecular signaling cascades, but little is yet known about the
negative regulation of plant innate immunity.

"Characterization of new host sensors will pave the way to inter-specific
and inter-generic transfer between plants of engineered receptors that
confer resistance to a variety of pathogens," Ronald says, adding that this
approach has already been demonstrated in transference work with cultivated
rice and wheat varieties, as well as with tobacco and tomato. "There may
also be room to engineer resistance in vertebrates as well, including
humans," she says.

In the Science paper, Ronald speculates that some microbes might
be pathogenic to humans because they have managed to evade detection by
human Toll-like receptors. Now that some of the essential building blocks of
immunity have been elucidated, she believes it may be possible to manipulate
these receptors so that microbes can no longer evade them.